Helper function for inserting external script tags

insertScript.js

/**
 * Inserts a script tag of src
 * in the Head of HTML.
 *
 * @param {string} src - Src for Javascript request
 * @param {Object} config
 * @return {Promise} - Returns a promise of event when loaded / error
 */
export default function insertScript(src, { async = true, defer = false } = {}) {
    return new Promise((resolve, reject) => {
        const gptScript = document.createElement('script');
        gptScript.async = async;
        gptScript.defer = defer;
        gptScript.type = 'text/javascript';
        gptScript.src = src;
        gptScript.onload = resolve;
        gptScript.onerror = reject;

        // Fetch!
        document
            .getElementsByTagName('head')[0]
            .appendChild(gptScript);
    });
}